Time-Delay-Aided Fast Beam Training for Near-Field Wideband XL-MIMO Systems

Extremely large-scale multiple input multiple output (XL-MIMO) plays an important role in providing ultra-high data rates in high-frequency millimeter-wave (mmWave) and Terahertz (THz) spectrum. Efficient beam training is essential as an enabler to ultra-high data rate transmission. However, the challenges brought by large array apertures and broad bandwidth, such as near-field propagation modeling and wideband near-field beam squint effect, make XL-MIMO systems suffer from high training overhead when carrying out beam training. To address this problem, based on the true-time-delay (TTD) deployment, we propose a frequency-scanning-based beam training scheme by exploiting the near-field channel characteristic for XL-MIMO systems. Firstly, we elaborate on the surrogate distance-angular two-dimensional representation for near-field beams and the corresponding squint effect. Then, a TTD-based 2-phase frequency-scanning scheme is proposed for overhead reduction. Numerical simulations verify that the proposed scheme can realize a comparable sum rate with low training overhead.